Permeability evolution in mechanically- and chemically-perturbed media using a granular mechanics model of EGS reservoirs

The evolution of permeability and porosity with time is an important feature of systems pushed far-from equilibrium - as represented by the strong mechanical and chemical disequilibrium developed in the stimulation of EGS reservoirs. In these systems, permeability responds to the evolution of large changes in effective stress in the short term and as a result of thermal stresses and chemical effects in the longer term. To explore the implications of these processes, especially related to short-term stimulation, we assemble critical processes in a granular mechanics model for porosity evolution. This approach represents the mechanical response of the fractured rock mass as a granular medium, accommodated through a form of "synthetic rock mass" to which appropriate thermo-hydraulic-mechanical-chemical couplings are included. Fluid flow is accommodated through a conduit model of flow between intervening pores. Fluid pressures feedback to mechanical deformation and mechanical deformation influences undrained fluid pressures. Mineral mass may be mobilized in the fluid stream by free-face dissolution or through mechanically mediated effects. The dissolved mineral mass is advected through the system and may be removed from the fluid by precipitation. Permeability in the 2D model is accommodated by an effective permeability that is modified in a dynamic manner by rates of dissolution driven by free-face or stress effects and by precipitation. The model has been validated for fluid, heat and mass transport in steady state and transient conditions.